This proposal investigates the ontogeny of basophils and mast cells, a high priority search area listed in the FOA. Basophils and mast cells are important components of type 2 immune responses, which cause allergic diseases and provide immune protection against parasitic infections. The incidence of allergic diseases, such as anaphylaxis, allergic rhinitis, atopic dermatitis, food allergy and asthma, has tripled in the Western countries since the 1980s. Hundreds of millions of people in the developing world are infected with helminth parasites. How basophils and mast cells develop into cells with specialized functions remains poorly understood. We have identified a novel population of common basophil-mast cell progenitors in the bone marrow. We have identified C/EBP? as the critical basophil transcription factor for specifying basophil cell fate and MITF as the crucial transcription factor for specifying mast cell fate. We demonstrate that transcription factor GATA2 is an upstream regulator of the Mitf and Cebpa genes. We found that MXD1 is a critical C/EBP? downstream transcription factor that might be important in basophil development and function, whereas BHLHE40 and AHRR are crucial MITF downstream transcription factors that might be important in mast cell development and function. Based on our preliminary analyses, we hypothesize that GATA2 upregulates the Mitf or Cebpa gene expression stochastically during the differentiation of pre-BMPs into basophils or mast cells and C/EBP?- dependent MXD1 plays a critical role in basophil development, whereas MITF-dependent BHLHE40 is crucial in mast cell development. We will test these hypotheses in 3 aims. In aim 1, we will analyze how GATA2 regulates the expression of the master basophil transcription factor C/EBP? gene and master mast cell transcription factor Mitf gene to specify the basophil and mast cell fates. In Aim 2, we will analyze the role of MXD1 in basophil development and function. In Aim 3, we will define the role of BHLHE40 and AHRR in MC development and function. Our grant proposal through innovative approaches will generate novel concepts and will significantly advance our understanding of how type 2 innate effector cells develop at the molecular level. The advancements in understanding basophil and mast cell fate determination is a critical concept, which must be better understood before subsequent studies to develop new treatment strategies or preventative methods for food allergy can be undertaken.